Special Senses

wizzstuffingUrban and Civil

Nov 16, 2013 (4 years and 1 month ago)

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Special Senses

Part 2

Light


Electromagnetic radiation


all energy
waves from short gamma rays to long
radio waves


Our eyes respond to a small portion of
this spectrum called the visible spectrum


Different cones in the retina respond to
different wavelengths of the visible
spectrum

Light

Figure 15.10

Refraction and Lenses


When light passes from one transparent
medium to another its speed changes and
it refracts (bends)


Light passing through a convex lens (as in
the eye) is bent so that the rays converge
to a focal point


When a convex lens forms an image, the
image is upside down and reversed right
to left

The spoon in this glass seems to be broken

When light passes from one transparent
medium to another its speed changes and it
refracts (bends)

Light passing through a convex lens (as in the
eye) is bent so that the rays converge to a focal
point

Refraction and Lenses

Figure 15.12a, b

Focusing Light on the Retina


Pathway of light entering the eye: cornea,
aqueous humor, lens, vitreous humor, and
the neural layer of the retina to the
photoreceptors


Light is refracted:


At the cornea


Entering the lens


Leaving the lens


The lens curvature and shape allow for
fine focusing of an image

http://www.youtube.com/watch?v=CuVxujt9JX
8


Focusing for Distant Vision


Light from a
distance needs
little adjustment
for proper
focusing


Far point of
vision


the
distance beyond
which the lens
does not need
to change shape
to focus (20 ft.)

Figure 15.13a

Focusing for Close Vision


Close vision requires:


Accommodation


changing the lens shape by
ciliary muscles to increase refractory power


Constriction


the pupillary reflex constricts
the pupils to prevent divergent light rays from
entering the eye


Convergence


medial rotation of the eyeballs
toward the object being viewed

Focusing for Close Vision

Figure 15.13b

Problems of Refraction


Emmetropic eye


normal eye with light
focused properly


Myopic eye (nearsighted)


the focal point
is in front of the retina


Corrected with a concave lens


Hyperopic eye (farsighted)


the focal
point is behind the retina


Corrected with a convex lens

Problems of Refraction

Figure 15.14a, b

Photoreception:

Functional Anatomy of
Photoreceptors


Photoreception


process by which the
eye detects light energy


Rods and cones contain visual pigments
(
photopigments
)


Arranged in a stack of
disklike

infoldings

of
the plasma membrane that change shape as
they absorb light

Figure 15.15a, b

Rods


Functional characteristics


Sensitive to dim light and best suited for night
vision


Absorb all wavelengths of visible light


Perceived input is in gray tones only


Sum of visual input from many rods feeds into
a single ganglion cell


Results in fuzzy and indistinct images

Cones


Functional characteristics


Need bright light for activation (have low
sensitivity)


Have pigments that furnish a vividly colored
view


Each cone synapses with a single ganglion cell


Vision is detailed and has high resolution

Chemistry of Visual Pigments


Rhodopsin


Retinal

is a light
-
absorbing molecule


Synthesized from vitamin A


Two isomers: 11
-
cis

and all
-
trans


Opsins



proteins


4 types that will absorb different wavelengths
of light


Excitation of Rods


The visual pigment of rods is
rhodopsin


(
opsin

+ 11
-
cis

retinal)


Light phase


Rhodopsin

(vitamin A oxidation) breaks down into
all
-
trans

retinal +
opsin

(bleaching of the pigment)


Dark phase


All
-
trans

retinal converts to 11
-
cis

form


11
-
cis

retinal is also formed from vitamin A


11
-
cis

retinal +
opsin

regenerate
rhodopsin

Excitation of Rods



The visual pigment of rods is
rhodopsin


(
opsin

+ 11
-
cis

retinal)


Light phase


Rhodopsin

breaks down into
all
-
trans

retinal +
opsin

(bleaching of the pigment)

Excitation of Rods


Dark phase


All
-
trans

retinal converts to 11
-
cis

form


11
-
cis

retinal is also formed from vitamin A


11
-
cis

retinal + opsin regenerate rhodopsin


CH
3

C

C

H

H

H
2
C

H
2
C

C

C

CH
3

H

CH
3

C

H

C

CH
3

C

H

C

H

C

H

C

CH
3

C

O

H

C

H

C

C

H

H

H
2
C

H
2
C

H
3
C

C

C

C

CH
3

CH
3

H

C

H

C

C

O

C

H

C

H

C

C

C

H

C

CH
3

H

CH
3

H

Oxidation

Rhodopsin

Opsin

All

-
trans


retinal



2H

+2H

Reduction

Vitamin A

Regeneration of

the pigment:

Slow conversion

of all
-
trans

retinal

to its 11
-
cis

form

occurs in the pig
-

mented

epithelium;

requires
isomerase

enzyme and ATP.

Dark

Light

11
-

cis


retinal

All
-

trans

isomer

11
-

cis


isomer

Bleaching of the

pigment:

Light absorption

by rhodopsin

triggers a series

of steps in rapid

succession in

which retinal

changes shape

(11
-
cis

to all
-
trans)

and releases

opsin.

Figure 15.16


Photoreception


Photoreception

Bleaching and Regeneration of Visual
Pigments


Phototransduction



Light energy splits
rhodopsin

into all
-
trans

retinal, releasing activated
opsin


The freed
opsin

activates the G protein
transducin


Transducin

catalyzes activation of
phosphodiesterase

(PDE)


PDE hydrolyzes
cGMP

to GMP and releases it
from sodium channels


Without bound
cGMP
, sodium channels close,
the membrane hyperpolarizes, and
neurotransmitter cannot be released

Signal Transmission in the Retina

Figure 15.17a

Signal Transmission

Figure 15.17b

Phototransduction


Light energy splits
rhodopsin

into all
-
trans

retinal, releasing activated
opsin


The freed
opsin

activates the G protein
transducin


Transducin

catalyzes activation of
phosphodiesterase

(PDE)


PDE hydrolyzes
cGMP

to GMP and releases it
from sodium channels


Without bound
cGMP
, sodium channels close,
the membrane hyperpolarizes, and
neurotransmitter cannot be released

Phototransduction

Figure 15.18

Adaptation


Adaptation to bright light (going from dark
to light) involves:


Dramatic decreases in retinal sensitivity


rod
function is lost


Switching from the rod to the cone system


visual acuity is gained


Adaptation to dark is the reverse


Cones stop functioning in low light


Rhodopsin

accumulates in the dark and retinal
sensitivity is restored

Visual Pathways


Axons of retinal ganglion cells form the
optic nerve


Medial fibers of the optic nerve decussate
at the optic chiasm


Most fibers of the optic tracts continue to
the lateral
geniculate

body of the
thalamus

Visual Pathways


Other optic tract fibers end in superior
colliculi (initiating visual reflexes) and
pretectal nuclei (involved with pupillary
reflexes)


Optic radiations travel from the thalamus
to the visual cortex

Visual Pathways

Figure 15.19

Visual Pathways


Some nerve fibers send tracts to the
midbrain ending in the superior colliculi


A small subset of visual fibers contain
melanopsin (circadian pigment) which:


Mediates papillary light reflexes


Sets daily biorhythms

Depth Perception


Achieved by both eyes viewing the same
image from slightly different angles


Three
-
dimensional vision results from
cortical fusion of the slightly different
images


If only one eye is used, depth perception
is lost and the observer must rely on
learned clues to determine depth

Retinal Processing: Receptive Fields
of Ganglion Cells


On
-
center fields


Stimulated by light hitting the center of the
field


Inhibited by light hitting the periphery of the
field


Off
-
center fields have the opposite effects


These responses are due to receptor types
in the “on” and “off” fields

Retinal Processing: Receptive Fields
of Ganglion Cells

Figure 15.20

Thalamic Processing


The lateral geniculate nuclei of the
thalamus:


Relay information on movement


Segregate the retinal axons in preparation for
depth perception


Emphasize visual inputs from regions of high
cone density


Sharpen the contrast information received by
the retina

Cortical Processing


Striate cortex processes


Basic dark/bright and contrast information


Prestriate cortices (association areas)
processes


Form, color, and movement


Visual information then proceeds
anteriorly to the:


Temporal lobe


processes identification of
objects


Parietal cortex and postcentral gyrus


processes spatial location

Chemical Senses


Chemical senses


gustation (taste) and
olfaction (smell)


Their chemoreceptors respond to
chemicals in aqueous solution


Taste


to substances dissolved in saliva


Smell


to substances dissolved in fluids of the
nasal membranes

Sense of Smell


The organ of smell is the olfactory
epithelium, which covers the superior
nasal concha


Olfactory receptor cells are bipolar
neurons with radiating olfactory cilia


Olfactory receptors are surrounded and
cushioned by supporting cells


Basal cells lie at the base of the epithelium

Olfactory Receptors

Figure 15.21

Physiology of Smell


Olfactory receptors respond to several
different odor
-
causing chemicals


When bound to ligand these proteins
initiate a

G protein mechanism, which uses cAMP
as a second messenger


cAMP opens Na
+

and Ca
2+

channels,
causing depolarization of the receptor
membrane that then triggers an action
potential

Olfactory Pathway


Olfactory receptor cells synapse with
mitral cells


Glomerular mitral cells process odor
signals


Mitral cells send impulses to:


The olfactory cortex


The hypothalamus, amygdala, and limbic
system

G
olf

Receptor

Extracellular fluid

Cytoplasm

Odorant

Adenylate cyclase

Na
+

Ca
2+

GTP

GTP

GTP

GDP

cAMP

cAMP

ATP

1

2

3

4

5

Figure 15.22

Olfactory Transduction Process

Taste Buds


Most of the 10,000 or so taste buds are
found on the tongue


Taste buds are found in papillae of the
tongue mucosa


Papillae come in three types: filiform,
fungiform, and circumvallate


Fungiform and circumvallate papillae
contain taste buds

Taste Buds

Figure 15.23

Structure of a Taste Bud


Each gourd
-
shaped taste bud consists of
three major cell types


Supporting cells


insulate the receptor


Basal cells


dynamic stem cells


Gustatory cells


taste cells

Taste Sensations


There are five basic taste sensations


Sweet


sugars, saccharin, alcohol, and some
amino acids


Salt


metal ions


Sour


hydrogen ions


Bitter


alkaloids such as quinine and nicotine


Umami



elicited by the amino acid glutamate

Physiology of Taste


In order to be tasted, a chemical:


Must be dissolved in saliva


Must contact gustatory hairs


Binding of the food chemical:


Depolarizes the taste cell membrane,
releasing neurotransmitter


Initiates a generator potential that elicits an
action potential

Taste Transduction


The stimulus energy of taste is converted
into a nerve impulse by:


Na
+

influx in salty tastes


H
+

in sour tastes (by directly entering the cell,
by opening cation channels, or by blockade of
K
+

channels)


Gustducin in sweet and bitter tastes

Gustatory Pathway


Cranial Nerves VII and IX carry impulses
from taste buds to the solitary nucleus of
the medulla


These impulses then travel to the
thalamus, and from there fibers branch to
the:


Gustatory cortex (taste)


Hypothalamus and limbic system (appreciation
of taste)

Influence of Other Sensations on
Taste


Taste is 80% smell


Thermoreceptors, mechanoreceptors,
nociceptors also influence tastes


Temperature and texture enhance or
detract from taste

The Ear: Hearing and Balance


The three parts of the ear are the inner,
outer, and middle ear


The outer and middle ear are involved
with hearing


The inner ear functions in both hearing
and equilibrium


Receptors for hearing and balance:


Respond to separate stimuli


Are activated independently

The Ear: Hearing and Balance

Figure 15.25a

Outer Ear


The auricle (pinna) is composed of:


The helix (rim)


The lobule (earlobe)


External auditory canal


Short, curved tube filled with ceruminous
glands

Outer Ear


Tympanic membrane (eardrum)


Thin connective tissue membrane that
vibrates in response to sound


Transfers sound energy to the middle ear
ossicles


Boundary between outer and middle ears

Middle Ear (Tympanic Cavity)


A small, air
-
filled, mucosa
-
lined cavity


Flanked laterally by the eardrum


Flanked medially by the oval and round
windows


Epitympanic recess


superior portion of
the middle ear


Pharyngotympanic tube


connects the
middle ear to the nasopharynx


Equalizes pressure in the middle ear cavity
with the external air pressure

Middle and Internal Ear

Figure 15.25b